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Greylisting - What is it and how does it work?

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  • 14/01/2014 4:05 PM

Greylisting is a method used by mail servers to reduce the amount of spam that is delivered to a mailbox. In fact it has been known to reduce spam by upto 90%.

Before we go into the detail of how it works we first need to understand how an email is sent and received.

When anyone wants to send an email they need to connect to use mail server in order to send it (usually called an SMTP server). When they connect to that server it will then look at the email address of the person they are sending it to and will look up what server needs to receive the email. It will then connect to the server and there is a conversation between the two servers before the email is delivered. The conversation runs something along the lines of :

Sending Server: 'I have a message from this person and want to deliver it to this email address'
Receiving Server: 'Okay, I've checked my records and that email address is in my list, go ahead and deliver your message'

The sending server then passes the message through to the receiving server which then has the responsibility of delivering it to the right mailbox.

What greylisting does is introduce a pause in the process the first time a person tries to send a message to someone. The receiving server keeps records of everyone who has sent an email, so when it receives a message from someone who isn't in the list it will reject the message and the conversation runs more like this:

Sending Server: 'I have a message from this person and want to deliver it to this email address'
Receiving Server: 'I can't take that message right now, please come back in 5 minutes.'

It is then down to the sending server to try to send it again at a later time. When it does the receiving server will know that it is the second time the message has been submitted and allows it to be delivered as usual. The receiving server will then remember that person for any future emails they send so that they don't have to go through the same process again.

The way that this can reduce spam is that most spammers just want to send out as much email in as shorter time as possible. What they don't want is to have to have mail waiting in a queue on their server. Consequently if a spammer cannot send an email the first time, they will very often give up straight away.

When working properly this system introduces the minimum of delays and effectively reduces spam. What can sometimes happen, though, is the sending server can be configured to wait longer than 5 minutes, and we have seen occasioan where it has waited up to 12 hours, before resending the message.

If your email goes through a Host100 mail server and you are finding that there is an excessive delay when receiving email from someone then it will be down to how the sending server has been configured and unfortunately it is not something that we will be able to help with. You may need to get the person sending the email to contact their mail administrator to check their server's configuration.

If the delay causes you a problem, and you don't mind the prospect of more spam, then depending on what Host100 mail system you use it might be possible to turn it off:

  • If you use are using the Host100 Smartermail system ( then we can disable the greylisting for your email. Just log into the Client Area and submit a support ticket and we will process your request as soon as possible. Please note though, this will affect every mailbox on your domain, not just your email address.
  • If you are using the Host100 Mailguard system then we cannot disable greylisting on this service.

If you have any other queries or problems not covered by this article then please feel free to contact us and we will do our best to help.

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